A sintered compact of common precipitation hardening stainless steel powder prepared by sintering/shaping, for example, metal powder injection molding (MIM), powder metallurgy, or sintering/shaping by powder deposition (so-called three-dimensional shaping) cannot achieve enough hardness, for example, a hardness of 40 HRC (or 390 HV) or more in a H900 treatment (480° C., air cooling) on a stainless steel SUS630 in accordance with JIS G 4303 only after direct aging treatment i.e. heat treatment for enhancing age hardening at a low temperature of 400 to 600° C.
Although aging treatment of the sintered compact after solution heat treatment at about 1050° C. described in JIS G 4303 can achieve enough hardness, it also causes new problems such as an increased number of steps, generation of distortion resulting from the solution heat treatment involving heating and quenching, and removal of distortion through re-annealing or shape correction. Thus, this cannot be a practical solution. Accordingly, a practical precipitation hardening stainless steel powder is required which can provide high strength only by sintering and aging treatments, but no powder is found which satisfies such requirements at present.
JP-2011-21218A (PTL 1) discloses a method of preparing practical aluminum prototypes and products by direct sintering or melting-solidifying. JP2002-249805A (PTL 2) also discloses another method of producing a sintered compact in which powder material composed of inorganic or organic matter is irradiated with a laser beam to stack melted layers into a desired three-dimensional shape.
Furthermore, JP2004-124201A (PTL 3) discloses a laser beam lithography with metal powder in which the step of sintering a thin metal powder layer into any form desired by laser beam is repeated to fabricate a laminated three-dimensional shaped object, for example, a metal prototype part or an injection mold. These patent documents, however, are all directed to non-ferrous metal, but not directed to treatment of steel powder, especially high strength stainless steel powder.